1. Field of the Invention
The present invention relates to cutting tools and wear-resistant materials, such as bearing balls, which are formed of silicon-nitride-based sintered bodies.
2. Description of the Related Art
Conventionally, silicon nitride sintered bodies, which contain as a main component silicon nitride and have excellent strength, are used for cutting tools and wear-resistant parts (wear-resistant materials) such as bearing balls.
Recently, there has been proposed a technique for improving the wear resistance of a silicon-nitride sintered body used as a cutting tool, through a reduction in the amount of a sintering aide (a certain type of oxide) added to the main component thereof (see Japanese Kohyo (PCT) Patent Publication No. 8-503664).
Separately, in order to realize a cutting tool having high wear resistance, the present inventors have studied a technique for controlling the hardness of a cutting tool in the vicinity of its cutting edge. However, conventionally, such control is not performed in practice.
Conventionally, when the Vickers hardness of a cutting tool is measured, an indentor is pressed against a substantially central portion of the rake face of the cutting tool in order to measure the hardness, and the hardness of the cutting tool in the vicinity of the cutting edge is not measured. This is because, since measurement of Vickers hardness requires some area, accurate measurement of the hardness of a cutting tool in the vicinity of the cutting edge has been impossible.